Field
The present invention relates to an acoustic transducer and a microphone. Specifically, the present invention relates to a capacitance type of acoustic transducer configured by a capacitor structure made up of a vibrating electrode plate (diaphragm) and a fixed electrode plate. The present invention also relates to a microphone that employs this acoustic transducer.
Related Art
FIG. 1A is a schematic diagram showing the structure of a conventional capacitance type of acoustic sensor (acoustic transducer). In an acoustic sensor 11 shown here, a cavity 13 is formed in a substrate 12 made up of a silicon substrate or the like, and a diaphragm 14 (vibrating electrode plate) is provided above the substrate 12 so as to cover the upper opening of the cavity 13. If the diaphragm 14 is rectangular, for example, the four corner portions thereof are supported on the upper surface of the substrate 12 by anchors 15. A dome-shaped back plate 16 is formed on the upper surface of the substrate 12, and the back plate 16 covers the diaphragm 14. A fixed electrode plate 17 is provided on the lower surface of the back plate 16, and the fixed electrode plate 17 opposes the diaphragm 14. Also, a large number of acoustic holes 18 that serve as passages for acoustic vibration are formed in the back plate 16 and the fixed electrode plate 17. Multiple stoppers 19 are provided on the lower surface of the back plate 16 so as to project from the fixed electrode plate 17. The stoppers 19 are provided in order to prevent the diaphragm 14 from sticking (adhering) to and not separating from the fixed electrode plate 17.
With this acoustic sensor 11, when acoustic vibration enters through the cavity 13, the diaphragm 14 vibrates due to the acoustic vibration (change in air pressure), and thus the distance between the diaphragm 14 and the fixed electrode plate 17 changes. The diaphragm 14 and the fixed electrode plate 17 oppose each other in a substantially parallel manner so as to configure a variable capacitor, and therefore when the diaphragm 14 vibrates due to acoustic vibration, the acoustic vibration is converted into change in the capacitance of the variable capacitor.
However, in this kind of capacitance type of acoustic sensor 11, the diaphragm 14 and the hack plate 16 are damaged if a large degree of pressure is applied to the diaphragm 14. Examples of situations in which a large degree of pressure is applied to the diaphragm 14 include the case where the diaphragm 14 is subjected to the pressure of air entering through the cavity 13 in a drop test performed on the acoustic sensor 11, the case where the device, such as a mobile phone, that includes the acoustic sensor 11 is dropped, the case where air is forcefully blown into the mouthpiece of a mobile phone that includes the acoustic sensor 11, the case where the mouthpiece is tapped by a finger or the like, and the case where shockwaves from a jet aircraft enter the cavity 13. If a large degree of pressure P is applied to the diaphragm 14 in this way, as shown in FIG. 1B, the diaphragm 14 bends a large amount due to the pressure P, and the diaphragm 14 collides with the back plate 16. Even if the diaphragm 14 bends, the pressure P applied to the diaphragm 14 does not escape, and therefore the diaphragm 14 and the back plate 16 undergo even larger deformation as shown in FIG. 1C. As a result, there are cases where the diaphragm 14 and the back plate 16 become damaged or cracked due to undergoing large deformation, and the damage resistance of the acoustic sensor 11 is poor.
Note that in the acoustic sensor disclosed in U.S. Pat. No. 6,535,460, the diaphragm is provided so as to cover the upper opening of the cavity, but the diaphragm is not fixed to the substrate. Also, the anchors are provided on the lower surface of the back plate. This acoustic sensor is structured such that when a bias voltage is applied between the diaphragm and the fixed electrode plate, the diaphragm is drawn upward due to the electrostatic attraction force, and the diaphragm is supported by the anchors.
Even with an acoustic sensor structured as disclosed in U.S. Pat. No. 6,535,460, the acoustic resistance is high in order to maintain the frequency characteristics, that is to say pressure is not likely to escape, and therefore when a large degree of pressure is applied to the diaphragm, a phenomenon such as that shown in FIGS. 1B and 1C occurs, and there is the risk of the diaphragm and back plate becoming damaged or cracked.
Also, the acoustic sensor disclosed in U.S. Pat. No. 8,111,871 is structured such that the diaphragm is supported by a spring, and when a large degree of pressure is applied to the diaphragm, the entirety of the diaphragm moves a large amount to allow the air pressure to escape.
However, with the acoustic sensor disclosed in U.S. Pat. No. 8,111,871, a spring suited to detecting acoustic vibration and a spring suited to allowing air pressure to escape cannot be designed independently, and there is a lack of flexibility in terms of design. Also, the spring is a linear member that is formed from the same material as the diaphragm and bent in a zigzag shape, and therefore the strength of the spring is low. Moreover, not only is the inertial force high since the entirety of the diaphragm moves, but also the back plate is not provided, thus leading to low strength with respect to a load other than pressure applied to the diaphragm, such as acceleration (inertial force) due to being dropped or external forces in the manufacturing process.
Also, with the acoustic sensor disclosed in US Application No. 2008/0031476, a plate-shaped deformation suppressing member (upper finger portion) is provided so as to oppose the edge of the upper surface of the diaphragm, and large deformation of the diaphragm is suppressed due to the diaphragm coming into contact with the deformation suppressing member when it undergoes large deformation.
Although deformation of the diaphragm can be suppressed in the structure disclosed in US Application No. 2008/0031476, the tip of the deformation suppressing member comes into contact with the edge of the diaphragm when the diaphragm undergoes large deformation, and therefore stress is concentrated at that location, and the strength of the diaphragm is likely to deteriorate.
U.S. Pat. No. 6,535,460, U.S. Pat. No. 8,111,871, and US Application No. 2008/0031476 are examples of background art.